......"Microengines have long been proposed but never successfully built due to the intricacies of feature sizes measuring down to 25 μm. The breakthrough came by exploiting developments in Micro Electromechanical System (MEMS) where miniscule metallic structures are fabricated to perform electrical, mechanical and optoelectrical functions. MEMS manufacture demands ±1-μm tolerances and Class 1000/100 cleanroom quality control.
Three variants of microengine have so far been made by the British researchers: a single-piston engine with a gear (see Figure 1(a)), a free-piston engine, and a micro-Wankel rotary engine. The single piston engine and the free piston engine are based on a two-stroke mechanism. These engines are designed to produce from 0.7 to 14 W.
Two-stroke was selected as it allowed the researchers to eliminate macroscopic features of a combustion engine, such as the valve train, that do not scale down well to the microscopic scale.
[Valves] used in four-stroke petrol engines, for example, would have been impossible to scale down accurately enough to be viable, explains the project leader, Dr Kyle Jiang. In a two-stroke engine, however, you can eliminate the need for [valves] by engineering it to be self-sealing so that one [inlet or exhaust port] closes as another opens up.
Similary, spark plugs have been impossible to replicate at the microscopic scale so the researchers exploited leading-edge materials science to engineer a chemical catalyst to burn fuel at the right moment.
The optimum type of fuel for the microengines is still undergoing research but will be based upon similar chemistries to conventional hydrocarbon fuels such as petrol and propane. Liquid CO2 has thus far been used."